Casting Resin

[Connectamabob]

I'm thinking also to get into resin cast, I have read is better to use acrylic resins than polyester resins, they don't release toxic vapours, they are less aggressive with the silicon mould, and because they are water based it's easier to clean the tools.

Seems acrylic resins are all advantages, but as far as i see everybody is using other kind of resins.
Anyone has experience on acrylic resins?

(raises hand) I've used acrylic resin. There's two basic kinds of acrylic resin: emulsion, and thermosetting. What you're describing is the former, but the latter is what's actually used for casting.

Acrylic emulsion is what's used as a media in acrylic paints, and is not at all suited to casting. It cures by having its carrier solvent evaporate out, and usually shrinks a lot (for a casting media) in the process. It also tends to be a bit weak and flexible (again, as a casting media) when cured. Imagine trying to cast an object out of solid PVA glue and you'll have an approximate image of why acrylic emulsion isn't used for casing.

For casting you'd want a thermoset acrylic resin. Unlike most two part resins, the A and B components come as a liquid (very low viscosity, like water or alcohol), and a powder respectively. The stuff makes wonderful finished pieces, and is very easy to use and forgiving to work with, but it's also insanely expensive, produces nasty fumes, and is actually much harsher on molds than urethane or polyester resins. It's fantastic stuff, but its downsides make it kind of a special purpose material from both a personal hobby and kit production standpoint.

is any difference on detail level  and viscosity between each type of resin?

Not categorically. Mostly for casting you'll be dealing with one of three types or resin: polyurethane, polyester, or epoxy. The rest is mostly down to the specific manufacturers formulas and product lines. When people recommend different resins, you should take that as recommending a particular brand and/or formula, not as a blanket statement about that type of resin in general.

There are some generalizations that can be made:

Polyester resin is what fiberglassing resins and body fillers (like Bondo) are made from. These are the cheapest and easiest to get, and are fairly easy to use. These are popular among garage level gaming terrain & vehicle producers for this reason. Polyester resins shrink more and take detail less precisely though, so they're best used for things where exact shapes are somewhat negotiable, and are not suitable for figures or anything that requires higher detail fidelity or precise part fit. The resin by itself tends to be brittle, and needs fillers to strengthen it. Their relatively low precision means you rarely see them used much outside of the gamer-targeted market.

Polyurethane is the most common casting resin type. There are many brands and formulas with many different working and finished characteristics, so these are the most versatile. Generally they are stronger than polyester, and take detail better than either polyester or epoxy. They are also more expensive than polyester, and require more careful attention during mixing and casting. Clear polyurethanes in particular are notorious for requiring very specific procedures and conditions in order to turn out properly. These are what you want to use if you're making figures, or vehicles/structures with higher and more precise levels of detail. Do your research: look at the companies' websites, and read over the material specs for the different product/formula lines before buying, as these do vary a lot. Some will be brittle as sugar, and others tough as iron, some will be thick as honey, and others will pour like water, and everything in between. Polyurethanes can make for more detailed and more durable models than most any other material, but you have to use the right one for the right job: most of the rep resin has for being less durable than metal and suchlike is down to manufacturers who weren't being choosy enough with their materials.

Epoxies are mostly used for protective clearcoats ("finishing epoxies"), higher performance fiberglassing, and of course glues and putties. There's more variety to them than polyesters, but lass than urethanes, and they tend to be more expensive than either. Aside from fiberglassing or production-level vacuuform molds, you don't see them used as casting resins very much. You can use then as such though. They tend to be rather on the viscous side, but that can be improved a lot by cutting them a bit with alcohol during mixing, and they can even be tinted using ordinary paints. These will be the least damaging to molds, and most of the consumer level ones will be less toxic than the others (though they are a sensitizer material, meaning over exposure can cause you to become allergic to them). Epoxies also make for very durable castings, though if you pick right, that's easy to match with urethane. They can take detail better than polyester, but not as good as a higher performance urethane.
 
Acrylic casting resins are the most expensive by a wide margin. They take detail as well as mid range urethanes, but they shrink a bit more and are much harsher chemically. They are impressively durable, but are also very easy to sand and scribe, and are the easiest and most forgiving of the lot to mix & cast.

Again, though, these are very much generalizations. Individual brands and formulas will vary a lot, and so will the needs of different molds/castings, so your need to take any recommendation for or against a particular resin as pertaining to that specific brand and/or formula, for that specific use.

Also your choice of mold rubber has to be matched to your choice of resin. There are many different brands of mold rubbers too, and they will give better or worse results for different resins and different shapes. Some types or rubber won't play well with some types of resin chemically, causing the resin to set up gooey. The best thing is to buy your rubber and resin from the same manufacturer, and read the documentation beforehand to make sure they're compatible, and that the rubber is the right hardness and flexibility for the size & shape of the object you're casting.

[Furt]

Good things come to those who wait!

A very thorough and detailed reply - albeit a little late. 

I have since used the two-part liquid resin to great success and have also experimented with dental plaster, which is OK for simple "blocky" casts.

Regardless I'm sure others will find the reply very useful.

[Connectamabob]

In regards to vacuum/pressure casting, the setup cost is kind of high. Sourcing and buy the components separately is cheaper than buying a packaged setup kit, but it does involve more time and labor though, so  whether it's cheaper on the whole depends on whether you're factoring that into the cost. Expect to spend a minimum of $300 US to get started in pressure casting, and that's if you already have an air compressor.

You need the compressor, a vacuum pump, a pressure chamber, a clear alternate lid for the chamber, plus gauges, hoses and fittings. A small (two gallon) chamber and suitable vacuum pump will run you roughly $100 each. Fittings are cheap individually, but they add up fast, so expect to spend around $20-30 there. You'll need a piece of clear acrylic sheet for the vac lid, which will cost you anywhere from $15 to $40, depending on specs and thickness (I used a 1x12x12" piece of cell cast acrylic for mine, which was about $30 plus shipping. It probably didn't need to be that heavy duty, but I wanted a wide safety margin), plus another $15 or so for a drill bit and tap for the hose & valve fitting. I cut my lid gasket from a foam counter top traction mat, so that only cost me $4. I already had a compressor, but if you don't, then tack on another $100 for that.

If you're just casting parts for personal use, the best thing IMO is to just get a resin that sets up very slowly. That way you can spend time on each pour making sure that everything gets distributed throughout the mold properly, fish/knock out bubbles, etc. You can get very good castings that way alone, if you're resin allows you the time. IMO you only need a vacuum/pressure setup if your molds have really fussy details, or if you're casting in volumes large enough to make quicker setting resins necessary.

[Connectamabob]

Good things come to those who wait!

A very thorough and detailed reply - albeit a little late.  

I have since used the two-part liquid resin to great success and have also experimented with dental plaster, which is OK for simple "blocky" casts.

Regardless I'm sure others will find the reply very useful.

I've been a member here for less than a year. The original thread was before my time.

A sticky on casting would be a good idea IMO. It's something that's bound to come up repeatedly, and I know there's people around here who are much more experienced than I am who might contribute.

[Furt]

I've been a member here for less than a year. The original thread was before my time.

No offense meant Connectabob, I'm merely a bit envious your obvious knowledge of the subject was not available to me earlier. 

As stated a sticky on casting would be very useful.

[Donpimpom]

wow! thanks a lot Connectamabob! thats a clear an enlightening reply.

I found a seller not too far from my town with a nice assortement of resins and silicons
 http://tienda.resineco.com

they have acrylic and polyester resin but no trace of uretane ones, and something called jesmonite, they describe it as a kind of acrylic resin compound for mould casting, but looking at the manufacturer website it only display resin's architetural applications
http://www.stevensons-of-norwich.co.uk/service/jesmonite/

any feedback about the jesmonite?
these guys (resineco) also have a blog (only spanish version available, sorry!) with step by step tutorials of simple mould casting, using their products of course, but still interesting as a begginer guide IMO
how to make a silicon mould
http://resineco.blogspot.com/2010/04/como-hacer-un-molde-con-silicona-para.html
how to cast resin copies (using this Jesmonyte stuff)
http://resineco.blogspot.com/2010/04/copias-con-molde-de-silicona-y-resina.html

the vacuum chamber is completely out of my plans (and my budged) I was considering the fishing bubble option, or event to build some kind of vibrating surface (if that would help float the bubbles)

i agree some kind of sticky about resin casting will be really helpful

[Connectamabob]

The Jesmonite stuff is new to me. Looks interesting though. I dug about a bit online, and found that it's an acrylic/gypsum hybrid material. From what I gather, it's like a kind of plaster that uses an aqueous acrylic in place of water. Interesting concept, and I'd love to see more details on how stuff made from it turns out.

some more info:
http://www.abic.se/pdf/Jesmonite%20AC100%20User%20manual.pdf
http://www.artrocker.com/rrpedia/Jesmonite



The blog links are interesting. Good instructions, though there are a couple of things I'd have done differently. Single-part open faced molds are popular, but few garage manufacturers do it right IMO. You want the mold face to be nice and flat so that you can use a backing during casting.

In the case of the blog demo, I'd take a sheet of MDF or melamine board and drill holes for the magnets so the turtle sits flush, chop the lip of the plastic container down to the intended depth of the mold and drill two large holes in the bottom, then place the modified plastic container upside down over the turtle, and pour the rubber through one of the holes in the container bottom 'till it's full. After demolding, trim another couple mm of the lip of the plastic container (to account for slight shrinking of the rubber), and you've got a finished mold with a nice flat face, with the modded container serving as a mother mold.

Once that's done, trace the "footprint" of the original pattern (the turtle) onto a cheap polyethylene cutting board, and drill a hole in the middle of the tracing (the size of the hole depends on the size of the pattern/casting, for this I'd say maybe 1cm). This is the mold backing. It ensures an even, flat back on the finished casting, and the hole acts as a vent and reservoir to allow the resin to shrink as it cures (all resins shrink, some just shrink more or less than others) without distorting the flat back of the part.

For casting, set the mold in its dish cavity side up, pour in the resin 'till the resin surface bubbles above the mold surface, then press the backing down on top. Alternatively you can start with the backing on, and pour the resin through the vent hole.

Many garage manufacturers who use open faced molds don't use backings, so you get all kinds of over and under filled edges that need to be sanded or filled or otherwise repaired before they can be assembled. The only good excuse for this IMO is being a newbie.

[Connectamabob]

the vacuum chamber is completely out of my plans (and my budged) I was considering the fishing bubble option, or event to build some kind of vibrating surface (if that would help float the bubbles)

For vibration you can just leave the mold on top of a running clothes washer or drier. If the parts you're casting are really simple and your resin really fluid, you can even just knock it a couple times on the bench top.

Before you pour the resin, dust the inside of the mold thoroughly with talc. The same properties that enable silicone molds to be used without a mold release also make it very surface tension friendly, and that's one of the reasons bubbles can get trapped in details so easily. The talc will break the surface tension of the resin so it flows into the wee nooks and crannies of the mold easier.

[Hammers]

No offense meant Connectabob, I'm merely a bit envious your obvious knowledge of the subject was not available to me earlier. 

As stated a sticky on casting would be very useful.

I may tidy this up and put it in How to

[OSHIROmodels]

Good move that man 

cheers

James

[Ramshackle_Curtis]

I have a vacuum pump that is in parts in a box. Its not running at optimal performance, I thrashed it somewhat. I works, but doesnt ever get to a really low pressure. It gets to the point where it sucks the bubbles out, but its inconsistent and annoying.

However, it would be a great pump to start up with, and Im happy to send it out for nothing, just the postage costs. Im in the UK.

It needs to be reassembled, and I dont have the instructions!

[Sangennaru]

I have a vacuum pump that is in parts in a box. Its not running at optimal performance, I thrashed it somewhat. I works, but doesnt ever get to a really low pressure. It gets to the point where it sucks the bubbles out, but its inconsistent and annoying.

However, it would be a great pump to start up with, and Im happy to send it out for nothing, just the postage costs. Im in the UK.

It needs to be reassembled, and I dont have the instructions!

I shall be interested. how much should the shipping cost be?

[Donpimpom]

Many garage manufacturers who use open faced molds don't use backings, so you get all kinds of over and under filled edges that need to be sanded or filled or otherwise repaired before they can be assembled. The only good excuse for this IMO is being a newbie.

I have a newbie doubt, the use of backings sounds really cool idea. I suppose, while casting, you put the back after some bubble fishing, but even with the vent hole on the back this would make the expel of bubbles more difficult, is'n it?

[Hammers]

Now with a nice air compressor in my possession (capable of holding at least 8psi) I have been toying with the idea of acquiring a pressure chamber with an removable lid for plaster and resin casting. It seems like the easier alternative to remove bubbles. I saw a web page with a fellow building his own not to long ago. Can't remember where, forget my own head next.

[OSHIROmodels]

I'm going to making my own vac chamber soon as we now have a moveable compresser at work 

cheers

James